In a typical xerographic method for reproduction, electric charges are sprayed onto a photoconductive surface by means of a corona discharge. Following establishment of a uniform charge on the photoconductive surface, an image to be reproduced is formed by exposing the charged surface to a pattern of radiation to selectively discharge areas of the photoconductive surface thereby forming an electrostatic latent image. Thereafter, toner, which may be in the form of electrostatic powder, is applied directly to the photoconductive surface and adheres in the image regions through electrostatic attraction. The toner image is thereafter transferred from the photoconductive surface to a sheet of suitable material, such as paper, and fused thereto.
Apparatus utilizing the above image forming technique requires that the photoconductive layer be specially prepared to resist the abrasive effects of exposure to toner powder and operations associated with the application, transfer of the toner powder and removal of residual unwanted toner. These effects limit the useful life of the photoconductive medium and gradually degrade copy image quality. Furthermore, such machines do not produce multiple copies from a single exposure and particularly cannot produce multiple copies bearing two or more different colors.
Efforts to eliminate some of these drawbacks can be illustrated by reference to prior patents. For example, Dessauer U.S. Pat. No. 3,013,878 discloses the use of a flexible transfer material in the form of a continuous belt of insulating material applied to the surface of a photoconductive drum. The belt is uniformly charged and image-wise discharged so that an electrostatic image is formed on its surface. Thereafter, the belt is toned, separated from the drum and the toner image is transferred to a support sheet. The belt continues onto the photoconductive drum surface where it is again charged and selectively discharged for the production of further copies. Since the belt carries an electrostatic surface charge, only a single copy from a single exposure is possible.
Methods for preservation of the electrostatic image have been previously described, e.g., see "Xerography and Related Processes" by Dessauer and Clark, particularly section 14.4 relating to "Special Forms of Electrostatic Transfer" disclosing the use of a backing electrode for the copy paper as it is stripped from the photoconductive surface. Walkup U.S. Pat. No. 3,251,706 discloses an electrostatic process in which a copy sheet or web is applied to a photoconductive surface after formation of an electrostatic image thereon. Toner is applied to the copy sheet and, after the sheet is removed from the photoconductive surface, the toner is fused to the sheet. The photoconductive surface retains the xerographic electrostatic latent image. If paper is utilized for the final copy, it has sufficient lateral conductivity to cause image spread resulting in low quality reproduction. To overcome this drawback the patentees advocate maintaining a small spacing between the paper and the electrostatic image surface which, however, results in a decrease in density and resolution unacceptable by current standards.
The prior art has also appreciated that the surface of a photoconductive member can be protected by an insulating film, e.g., Butterfield U.S. Pat. No. 2,693,416, and various prior patents have disclosed the use of an insulative film integrally bound to a photoconductive surface, e.g., Schlein et al. U.S. Pat. No. 3,537,786 and Walkup U.S. Pat. No. 2,934,649. In Schlein et al., the patentees recognized the deficiencies of xerographic processes and utilized as the latent image storage means a medium capable of being internally polarized and which relies on the phenomenon known as "Persistent Internal Polarization." Other patents of interest herein include U.S. Pat. Nos. 62,044, 1,552,788, 1,706,182, 1,723,206, 1,784,912, 1,956,820, 2,221,776, 2,297,691, 2,357,809, 2,368,648, 2,618,551, 2,618,552, 2,624,652, 2,633,796, 2,732,775, 2,756,676, 2,811,465, 2,812,709, 2,825,814, 2,833,648, 2,975,052, 3,084,061, 3,128,683, 3,234,019, 3,254,998, 3,355,289, 3,429,701, 3,438,772 and 3,458,310.
The present invention provides an electrostatic process which overcomes the foregoing drawbacks and which enables the production of a plurality of copies on "plain" copy paper. The process also permits the production of full color reproduction in which two, three or more toner colors are overlayed onto a single copy sheet. Inexpensive photoconductive materials can be used, such as zinc oxide, as well as the traditional selenium or cadmium sulfide surfaces. In certain embodiments an insulative, non-photoconductive surface may be used. Importantly, the image carrying surface is completely protected from the abrasive effects of toner application and therefore has an extremely long lifetime. In broader terms, any means for developing the image such as differential melting, dimpling by heat, and the like, may be used.
The present invention utilizes inductive electrophotography in which an insulating film overlies an electrostatic image carrying surface and involves a unique combination of insulating film properties, charging levels and other process conditions to develop an image, as with toner or otherwise, while retaining an electrostatic latent image on the image carrying surface. In prior art processes, when toner is deposited directly on the charged photoconductor surface, some charge is lost directly to the toner. The interposition of a thin insulating film as described below, prevents loss of charge. In accordance with the present invention, a thin insulative member, such as a plastic film, is applied in direct contact with an insulative surface which may be photoconductive and which may be adapted for having an electrostatic image formed thereon. When using a photoconductive surface, it is initially electrostatically charged to a predetermined potential which is sufficient to effect development of an image on the insulating film but which is insufficient to discharge when the image is removed from the photoconductive surface. Before or after application of the insulating film the charged photoconductive surface is exposed to a pattern of radiation to form a latent electrostatic image of the pattern on the photoconductive surface. When using a non-photoconductive insulative surface, an electrostatic image is placed directly thereon by means of an electron beam or by a charged stylus, or the like. If toner is used as the developing agent, it can be applied while one surface of the insulating film is in direct contact with the image carrying surface, adhering to the opposite surface by induction. Thereafter, the toner may be transferred and this can occur while the insulating film is in contact with the photoconductive surface. Preferably, the insulating film is separated from the image carrying surface and the toner image is fused to the film or is transferred to a support member, such as copy paper, and then fused to the paper. Since the electrical field at the image carrying surface is insufficient to cause discharge upon separation of the insulating film, the latent image can be reused by reapplying the same insulating film or by applying a fresh length of insulating film. Neither further charging nor further exposure is required to produce another toner image on the free surface of the insulating film.
The insulating film can be in the form of a closed loop and the image carrying surface carried by a drum so that a plurality of copies can be continuously formed. If the circumference of the drum is equal to the length of one document, copies are produced continuously as the drum rotates and the endless loop continues to advance. If the drum circumference has a length equivalent to a plurality of documents, or if a belt or other suitable configuration of the image carrying medium is provided having such length, after one charge and exposure for each page, copies of a document of plural pages are produced in sequence. In a further embodiment where the drum surface is three or four documents long, exposure of each segment can be effected with successively different wavelengths of radiation and a different colored toner applied to each segment of insulating film in synchronization with rotation of the drum. The same copy sheet is repeatedly brought into successive contact with each segment for successive transfer of the differently colored toner images to a single sheet to thereby provide a full color reproduction of the document. Alternatively, the insulating film can constitute the final copy which may be either transparent or opaque. The thin insulating film can be laminated to another material for support and/or to protect the image.
As above indicated, certain parameters are critically important to operation of the processes of this invention, their combination yielding high quality copies which would otherwise not be produced. The thin insulating film should be a good electrical insulator, and be smooth, uniform and thin. Additionally, for reuse, the film should be dimensionally stable, highly abrasion and wear resistant and thermally stable. It is critical that the film be in intimate contact with the image carrying surface so that there is virtually no space between the insulating film and photoconductive surface. The insulating film should have high lateral electrical resistivity to prevent image spread, at least about 10.sup.13 ohms/.quadrature. of surface, and a maximum thickness of about 3.0 mils, preferably about 1.5 mils, to allow acceptable resolution and density of the toner image. Films as thin as 0.1 mil, preferably 0.25 mil, can be employed, although no minimum thickness need be specified. Additionally, the potential to which the photoconductor is charged should be insufficient to initiate charge transfer or discharge between the image carrying surface and the thin insulating film, generally about 800 volts. See, in this regard, Electrophotography by R. M. Schaffert, Focal Press, London, New York, 1965. Since the insulating film is never charged, except by induction, and the electrically imaged surface itself is not developed, the electrostatic image will not be removed. However, with photoconductors, their inherent decay characteristics will result in eventual dissipation. Accordingly, a photoconductive material should be selected having a decay rate sufficiently low to produce the number of copies desired. Photoconductive media composed of sensitized photoconductive particles dispersed in a polymeric matrix can be controlled to give extremely low decay rates, as can evaporated amorphous selenium, enabling the production of a multiplicity of high quality copies.
For purposes of simplicity of description, the following detailed description of the invention refers specifically to image development with dry toner. However, it will be appreciated that liquid toner application is fully equivalent and that in many embodiments other known methods for electrostatic image development can be substituted directly or with only minor modification, all well within the skill of the art.